In diagnostic ultrasonic imaging, several modes are known for processing electroacoustic return signals. These different modes have the purpose of displaying different types of tissues or objects that may be more or less echogenic.
Static tissues typically have a good echogenicity and are optimally detectable by ultrasonic imaging at the fundamental frequency by using B-mode imaging. In this mode, as is well known, the envelope of the return signal is detected, and the envelope amplitude is correlated to a grey scale, thereby forming an image. B-mode images at the fundamental transmission frequency are high-quality images, as the transmitted signal may consist of a broadband pulse, i.e. a very short pulse, which ensures an optimized resolution.
Static tissues, and particularly soft tissues, also generate nonlinear reflected signals. The generated return signal not only contains a fundamental component, i.e. at the same frequency as the transmitted beam, but also comprises nonlinear components, as components at the second or higher-order harmonics of the fundamental transmission frequency.
Highly perfused tissues or blood flows or other body flows are poorly echogenic, i.e. generate reflection contributions well below the intensity of the reflection contributions generated by static tissues. This limitation was accordingly obviated by using substances that amplify the reflection response and are introduced in the blood flows. These substances, known as contrast agents, have a strongly nonlinear reflection response, and therefore generate return signal contributions at the second harmonic of the transmitted beam fundamental frequency. The harmonic imaging mode, developed for contrast agent ultrasonic imaging, removes the fundamental frequency contributions of the return signal and used the second harmonic components generated by contrast agents for image reconstruction.
Nevertheless, as mentioned above with respect to soft tissues, these tissues also generate nonlinear contributions to the return signals, such that it is difficult or impossible to differentiate between return signals in blood flow from soft tissue and return signals due to the presence of contrast agents.
U.S. Pat. No. 6,066,098 teaches an ultrasonic imaging method which does not only use a certain frequency of the return signal, but acquires the whole spectrum of the received signal frequencies. As this spectrum is characteristic for each type of tissue, this method permits the identification of the type of tissue that generated the reflected signal at a predetermined scanning depth and to differentiate tissue contributions from contrast agent contributions in the return signal.
From documents U.S. Pat. No. 6,290,647 and U.S. Pat. No. 6,117,082, it is known to use another characteristic of contrast agents to differentiate the contrast agents from tissue in ultrasonic imaging. In deed, when the transmitted signal is exposed to an appropriate mechanical pressure, contrast agent return signals may be excited, thereby creating signal components at a subharmonic of the transmitted signal frequency. Such subharmonic frequency is not substantially present in the reflected signals generated by physiological tissues. The two documents show that ultrasonic imaging of regions that carry contrast agents may be performed by using components of the subharmonic signal, alternatively or in addition to the harmonic component of the return signal.
Nevertheless, the excitation of the subharmonic components of the return signal generated by contrast agents requires the transmitted pulses to be relatively long, and such time length of transmitted pulses causes an image quality degradation in terms of resolution, as well as the presence of side lobes and generation of artifacts. On the other hand, traditional B-mode images, obtained by using the linear component of the return signal permits, as mentioned above, the generation of high quality images.
It shall be further noted that the use of contrast agent signals is not generally designed for B-mode imaging, but for imaging in a narrow sense, and especially for detecting the presence of contrast agents and possibly the perfusion characteristics thereof, or for determining the speed or amount of flow.
Due to the above, the methods taught by U.S. Pat. Nos. 6,290,647 and 6,117,082 involve a number of image quality restrictions. Problems also arise in the excitation and reception of subharmonic signals, as the band widths of ultrasonic probes are generally insufficient to ensure the reception of harmonic and subharmonic frequencies of the fundamental transmission frequency.